The subject matter of the present invention relates to a firing system adapted for use in a perforating apparatus, and more particularly, to a differential pressure firing system disposed in a perforating apparatus adapted for use in a wellbore, the firing system maintaining the pressure above and below a firing apparatus pressure balanced at a rathole (hydrostatic) pressure and, by means of a plurality of sequentially operable hydraulic systems, maintaining the balanced rathole pressure across the firing apparatus until a packer is set and a pressure above the packer is increased to a level which exceeds the rathole hydrostatic pressure.
The concept of using differential pressure to fire a perforating apparatus is not new. For example, U.S. Pat. Nos. 4,817,718 and 4,880,056 to Nelson et al and U.S. Pat. No. 4,606,409 to Peterson et al disclose differential pressure fired perforating systems. In general, existing differential pressure fired perforating systems use the difference between annulus pressure above the packer and tubing pressure to power either a mechanical system or a hydraulic system, that is, an operating piston above the packer is always applying the annulus pressure above the packer to a side of a firing piston or a mechanical actuator; however, any movement of the firing piston or actuator is prevented by a tubing pressure being applied directly to the other side of the firing piston or mechanical actuator. However, in these existing systems, safety requires that the above the packer annulus pressure be maintained equal to the tubing pressure until such time that detonation of the perforating system is required. Only then can the annulus pressure above the packer be increased to a level greater than the tubing pressure below the packer.
One existing system is disclosed in U.S. Pat. Nos. 4,817,718 and 4,880,056 to Nelson et al. In order to maintain a margin of safety when using the existing system disclosed in the Nelson et al patents, a reference pressure chamber, located within the confines of the tool, is physically disposed below the firing piston and is filled with fluid at hydrostatic pressure; the annulus pressure above the packer is increased to a first pressure level; this closes off the reference pressure chamber and traps the fluid therein at the hydrostatic pressure thereby maintaining the pressure below the firing piston at the hydrostatic pressure; the annulus pressure above the packer is increased further to a second pressure level; this increases a pressure above the firing piston to a level above the hydrostatic pressure in the reference pressure chamber thereby creating a differential pressure and driving the firing piston onto a primer which detonates the perforating apparatus. However, since the reference pressure chamber is located within the confines of the tool, the firing piston is not open, at both ends, to an annulus area below the packer (also termed the "rathole annulus") prior to application of the differential pressure across the firing piston. Therefore, premature detonation of the Nelson system could occur because the Nelson firing piston is not truely pressure balanced prior to detonation. Since the firing piston is not open at both ends to the rathole annulus, the Nelson et al differential pressure firing system is classified as a "closed" system.
Still another existing system is disclosed in U.S. Pat. No. 4,606,409 to Peterson et al. At least for a short period of time, the system disclosed in Peterson et al may be considered an "open" system; that is, for a short period of time, the top and bottom ends of the firing piston are exposed to fluid pressure which exists within the rathole annulus. A check valve admits pressurized fluid into a pressure chamber disposed near the top part of the firing piston but immediately closes when the pressure chamber is full thereby trapping the fluid therein at hydrostatic pressure. However, when the check valve closes, since the top part of the firing piston is closed relative to the rathole annulus, the system is no longer considered to be an "open" system; rather, it is a "closed" system, one which may be subject to premature detonation due to unwanted pressure differences which exist between the top and bottom ends of the firing piston. Therefore, in order to maintain a margin of safety when using the system disclosed in Peterson, a locking sleeve piston prevents any premature movements of the firing piston toward a primer charge. Peterson's firing piston is actuated by bleeding off the pressure in the tubing string below the packer when the check valve is closed thereby reducing the pressure below the firing piston relative to the pressure existing within the pressure chamber.
However, none of the aforementioned existing systems are, at all times, open to the rathole annulus immediately prior to creating the differential pressure and actuating the firing piston; therefore, none of the existing systems may be truely classified as an "open" system. However, in-any event, none of the, existing systems maintain the firing piston or mechanical actuator pressure balanced at the rathole hydrostatic pressure by "opening" both ends of the firing piston or actuator to the rathole annulus below the packer and keeping both ends of the firing piston open to the rathole annulus until the packer is set and the pressure in the annulus above the set packer is increased to a point which exceeds the rathole hydrostatic pressure. In addition, the system disclosed in the Nelson et al patents requires that the distance between the packer and the firing head be limited in order to maintain the required differential pressure across the firing head which is necessary to detonate the firing head.